1. Field of the Invention
The present invention relates to a sensing device as well as to a related sensing method.
In particular, the invention relates to at least one level sensor, i.e. a device and/or an electronic circuit adapted to detect the presence and/or level of a substance, e.g. a fluid preferably of the electrically conducting kind, in particular at a certain height above a reference surface, such as the detection of a conducting liquid at a certain height in a container used for collecting it.
2. Present State of the Art
The invention preferably and advantageously applies to filters for vehicles, in particular to fuel filters.
Several devices for sensing the presence or level of substances are known in the art, which operate on the basis of different physical principles.
Some of these devices utilize an electrode pair and detect the presence of the substance based on the variation in the electric resistance across the electrodes.
For instance, patent U.S. Pat. No. 4,276,161 discloses a fuel filter fitted with a level sensor capable of sensing the level of water collected in a chamber of the filter itself. In this solution, an electrode is connected to the input of a comparator and its free end is immersed into the water collection chamber. When the water level reaches the electrode, the water establishes an electric connection between the filter body (which acts as an electrode and ground terminal) and the electrode, thus causing the comparator output to change and allowing the presence of the liquid to be detected.
However, this solution suffers from the drawback that the electrodes will degrade prematurely due to the fact that the direct voltage applied across the electrodes gives rise to electrochemical phenomena, such as erosion phenomena, when the electrodes are immersed into the water.
As an alternative to direct voltages, according to a known solution the electrodes are supplied with an alternating voltage generated by an oscillator supplied with a direct voltage, like that provided by a vehicle's battery. A similar solution is known from U.S. Pat. No. 3,978,463, which discloses a level indicator for vehicles which detects the absence of fuel in a tank.
In patent U.S. Pat. No. 3,978,463, the oscillator's alternating signal is applied directly to a switch, consisting of a transistor, which changes its state depending on the presence or absence of fuel. In this patent, the oscillator's alternating signal is applied directly to a switch, consisting of a transistor, which changes its state depending on the presence or absence of fuel. In this case as well, it may happen that direct voltages are present across the electrodes, e.g. because of electrostatic discharges which are detrimental to electrodes.
In both solutions known from U.S. Pat. Nos. 4,276,161 and 3,978,463, one electrode consists of the very container which collects the conducting liquid and which is used for providing the electric ground connection. This container is usually connected to other components of the vehicle, which are adapted to support and secure it. Therefore, with these solutions there is a risk of anomalous electric ground contacts, e.g. with the creation of anomalous electric resistances, also called “parasitic resistances”, in series with the electric power circuit; this phenomenon, also referred to as “ground offset”, may lead to wrong readings, e.g. due to anomalous electric voltages added to the voltage corresponding to the actual measured level. This phenomenon may also be found in other cases, as in an electric wire harness including electric connectors with oxidized contacts.
The aforementioned types of sensing devices, in particular level sensors for vehicular filters, are typically associated with electric or electronic circuits having a very simple structure, e.g. a lamp that goes on when water is detected within a fuel filter (U.S. Pat. No. 4,276,161), or they may be associated with rather complex electronic circuits, such as electronic circuits with a microprocessor or a microcontroller capable of controlling actuators, e.g. valves or pumps, as a function of the signal detected by the sensor. For example, US2006/0070956 discloses a level sensor connected to a microprocessor that drives an electromagnetic pump in order to drain the water collection container of the fuel filter when it receives the maximum level signal from the sensor.
Patent U.S. Pat. No. 6,763,713 discloses that a vehicle's control unit may be suitably programmed for detecting a fault in the fuel level sensor. This detection is possible by cross-referencing the data obtained from the level sensor with other information received from other different sensors.
The known types of detectors, in particular level sensors, more in particular for filters for vehicles, include very simple circuits that, although quite inexpensive, cannot carry out complex functions or are associated with relatively sophisticated circuits, which are however more expensive.